In conventional cellular telephone networks, a base station is provided for each cell of the area served by the cellular network. Each base station comprises a plurality of radio transceivers which provide radio channels for voice communications between the base stations and mobile telephones in the cells served by the base stations. The base stations are connected to mobile switching centers which provide telecommunications switching between base stations. A gateway mobile switching center is connected between the mobile switching centers and a Public Switched Telephone Network (PSTN) so that mobile telephones served by the cellular telephone networks can be connected to telephones served by the PSTN.
In addition to voice telephony services provided to mobile telephone users by cellular telephone networks, there is a demand for packet data services provided to mobile data terminals. In April 1992 an industry consortium was formed to develop standards for providing Cellular Digital Packet Data (CDPD) services. In July 1993 this consortium released version 1.0 of a CDPD Specification which defines standard interfaces and functionality for CDPD networks. Version 1.0 of the CDPD Specification is hereby incorporated by reference.
A CDPD network may be implemented as an overlay on an existing cellular telephone network. The CDPD Specification calls for Mobile Data Base Stations (MDBSs) to serve mobile data terminals called Mobile End Stations (MESs). The MDBSs are connected to Mobile Data Intermediate Systems (MDISs) which are connected to external public or private Packet Data Networks (PDNs) so that the MESs can exchange packet data with Fixed End Stations (FESs) connected to the PDNs.
The MDBSs use the same radio frequency channels to exchange packet data with the MESs as do voice base stations serving mobile telephones in the same serving area. To avoid radio interference bet-ween packet data transmissions and voice transmissions, the MDBSs must use radio frequency scanners to scan the voice channels to determine which voice channels are currently in use by the voice base stations serving the same area, and tune their transceivers to only those channels which are not currently in use for voice communications. Consequently, the MDBSs "hop" among the voice channels to avoid voice calls which are currently in progress.
The frequency scanning and retuning operations of the MDBSs require considerable processing. Moreover, each frequency hop executed in order to "dodge" a voice call interrupts packet data transmission, reducing the data throughput of the CDPD network. Furthermore, because expensive MDBS hardware, MDIS hardware and transmission facilities linking the MDBS hardware to the MDIS hardware are needed to provide CDPD service, the cost of introducing CDPD service is higher than desired, particularly where the initial demand for CDPD service is limited. If the CDPD service providers price the service high enough to pay back their equipment investment quickly or limit deployment of CDPD service to high traffic areas, they risk limiting CDPD market growth.
Moreover, the boundaries of cells served by MDBSs do not coincide exactly with the boundaries of cells served by voice base stations even when the MDBSs and the voice base stations are co-located. The cell boundaries do not coincide exactly because the intercell hand off criteria are different for voice and packet data transmission. The mismatch of cell boundaries can lead to excessive interference between channels used for voice communications and channels used for packet data communications.